UN said that it will help an estimated 1.7 million people in hard-to-reach areas by the end of March.
A cessation of hostilities began on Saturday and there have been complaints of breaches from both sides. But it otherwise appears to be intact with a key Syrian opposition group saying the situation was much better.
Yacoub el-Hillo, the UN’s humanitarian coordinator in Syria, called the truce “the best opportunity that the Syrian people have had over the last five years for lasting peace and stability”.
The organization plans to use the lull to deliver food, water and medicine to towns like Madaya, where residents have reportedly been starving to death.
It said it needs approval of Syria’s warring parties before it could further expand its deliveries.
Efforts to deliver aid to Islamic State-besieged Deir al-Zour by air last week failed when several pallets were damaged, disappeared or landed in no-man’s land.
The UN estimates almost 500,000 people are living under siege in Syria.
The cessation of hostilities was agreed as part of a US-Russian plan, which have backed opposing sides in Syria’s civil war.
It does not apply to the fight against so-called Islamic State (IS) or the al-Qaeda-linked Nusra Front.